Due to environmental pollution and oil energy depletion, world-wide studies on environment-friendly electric vehicles (EV) are going along. As demands for and developments on EVs and plug-in hybrid vehicles (PHEV) increase, an on-board charger (OBC) for high-voltage battery charging becomes an essential component in automotive industry. Meanwhile, instead of conductive charging in which connectors are used, wireless power transfer (WPT) technologies used for charging high-voltage batteries without connectors have been introduced.
In a wireless charging system for EV charging, a primary pad and a secondary pad can be modeled as a transformer in an equivalent circuit. As compared with conventional converter transformers, a coupling coefficient is relatively low since an air-gap between the primary pad and the secondary pad is very large. That is, since magnetizing inductance is much larger than leakage inductance, it may become difficult to transfer power to an output. Therefore, a method, in which at least one capacitor is applied to the primary pad and secondary pad so that a resonance between the pads is caused by inductance and capacitance of the pads, is used usually.
Usually, a phase-shifted full-bridge converter is used as a converter for the wireless power transfer system, which is connected to a front end of the primary pad. Also, a zero voltage switching (ZVS) technique is used for improving efficiency of the converter. It is not so difficult to design the converter to achieve ZVS by using capacitances of the primary and secondary pads.
However, misalignment between the primary and secondary pads, manufacturing tolerances of the pads and capacitors, and different characteristics of them may break their resonance during wireless charging. Also, in this reason, inverse-currents flowing through switches in the converter may be generated excessively, ZVS of the converter may not be guaranteed, and efficiency of the wireless power transfer system may degrade rapidly.
Also, switches having large current capacity should be used in order to cope with the inverse-currents. In addition, if the ZVS is not guaranteed, increase of electromagnetic interference (EMI) caused by hard switching may demand an additional filter for reducing the EMI, whereby the size and material cost of the total system are increased.